System and method for controlling heat flow in a downhole tool

ABSTRACT

An apparatus is disclosed for controlling heat flow in a downhole tool using a thermal rectifier material. The thermal rectifier material is positioned between a heat source and a heat sink for reducing flow of heat returning from the heat sink to the heat source. That is, the thermal rectifier conceptually operates as a “thermal check valve” so that heat, which has flowed (or been pumped) out of a region, has difficulty returning to that region. Another embodiment of an apparatus is disclosed for controlling heat flow in a downhole tool, which includes a thermal rectifier material surrounding a liquid supply, wherein the thermal rectifier material allows more heat to flow through the thermal rectifier in a first direction away from the liquid supply than in a direction through the thermal rectifier material toward from the liquid supply. A method for controlling heat flow in a downhole tool using a thermal rectifier material is also disclosed.

BACKGROUND

1. Field of the Invention

The field of the present invention relates to down hole tools and in particular to controlling heat flow in down hole tools.

2. Background of the Related Art

In underground drilling applications, such as oil and gas or geothermal drilling, a borehole is drilled through a formation deep into the earth. Such bore holes are drilled or formed by a drill bit connected to the end of a series of sections of drill pipe, so as to form an assembly commonly referred to as a “drill string”. The drill string extends from the surface to the bottom of the borehole. As the drill bit rotates, it advances into the earth, thereby forming the borehole. In order to lubricate the drill bit and flush cuttings from the drill bit's path as it advances, a high pressure fluid, referred to as “drilling mud”, is directed through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through an annular passage formed between the exterior of the drill string and the surface or interior wall of the bore hole.

The distal or bottom end of the drill string, which includes the drill bit, is referred to as a “downhole assembly”. In addition to the drill bit, the downhole assembly often includes specialized modules or tools within the drill string that make up an electrical system for the drill string. Such modules often include sensing modules. In many applications, the sensing modules provide the drill string operator with information regarding the formation as it is being drilled through, using techniques commonly referred to as “measurement while drilling” (MWD) or “logging while drilling” (LWD). For example, resistivity sensors may be used to transmit and receive high frequency signals (e.g., electromagnetic waves) that travel through the formation surrounding the sensor.

As can be readily appreciated, such an electrical system may include many sophisticated electronic components, such as the sensors themselves, which in many cases include printed circuit boards. Additional associated components for storing and processing data in the control module may also be included on printed circuit boards. Unfortunately, many of these electronic components generate heat. For example, the components of a typical MWD system (i.e., a magnetometer, accelerometer, solenoid driver, microprocessor, power supply and gamma scintillator) may generate over 20 watts of heat. Moreover, even if the electronic component itself does not generate heat, the temperature of the formation itself typically exceeds the maximum temperature capability of the components.

Overheating downhole frequently results in failure or reduced life expectancy for thermally stressed electronic components. For example, photo multiplier tubes, which are used down hole in gamma scintillators and nuclear detectors for converting light energy from a scintillating crystal into electrical current, may not operate above 175° C. Consequently, cooling of the electronic components can be important. Unfortunately, cooling is made difficult by the fact that the temperature of the formation surrounding deep wells, especially geothermal wells, is typically relatively high, and may exceed 200° C.

Thus, one of the prominent design problems encountered in downhole logging tools is associated with overcoming the extreme temperatures encountered in the downhole environment. Thus, there exists a need to reduce the temperature within the downhole tool in the region containing the electronics, to the within the safe operating level of the electronics. Various schemes have been attempted to resolve the temperature differential problem to keep the tool temperature below the maximum electronic operating temperature, but none of the known techniques have proven completely satisfactory.

Downhole tools are exposed to tremendous thermal strain. The downhole tool housing is in direct thermal contact with the borehole drilling fluids and conducts heat from the borehole drilling fluid into the downhole tool housing. Conduction of heat into the tool housing raises the ambient temperature inside of the electronics chamber. Thus, the thermal load on a non-insulated downhole tool's electronic system is enormous and can lead to electronic failure. Electronic failure is time consuming and expensive. In the event of electronic failure, downhole operations must be interrupted while the downhole tool is removed from deployment and repaired.

SUMMARY

A thermal rectifier material or device is provided in a downhole tool. The thermal rectifier material or device that controls heat flow in the downhole tool so that heat flows more easily in a first direction, away from a heat source such as an electronics package in the tool toward a heat sink, than in a second direction, away from the heat sink and toward the heat source. The thermal rectifier material is useful in cooling systems that remove heat from heat sources such as electronics in downhole tools.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an depiction of a particular illustrative embodiment in a monitoring while drilling environment;

FIG. 2 is a longitudinal cross section through a portion of the down tool attached to the drill string as shown in FIG. 1 incorporating a thermal rectifier material in combination with a sorption cooling apparatus or a thermoelectric cooling device;

FIG. 3 is a transverse cross section through one of the sensor modules shown in FIG. 2 taken along line III-III;

FIG. 4 is an depiction of an illustrative embodiment shown deployed in a wire line environment;

FIG. 5 is a schematic diagram of an illustrative embodiment showing heat flow when an thermoelectric cooling device is electrically powered, during a pulsed thermoelectric cooling device “on” cycle;

FIG. 6 is a schematic diagram of an illustrative embodiment showing heat flow when the thermoelectric cooling device is not electrically powered during a pulsed thermoelectric cooling device “off” cycle;

FIG. 7 is a schematic diagram of another illustrative embodiment showing a thermal rectifier material controlling heat flow into a thermal flask; and

FIG. 8 is a flow chart of functions performed in a particular illustrative embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In a particular illustrative embodiment an apparatus for controlling heat flow in a downhole tool is disclosed. The apparatus includes a thermal rectifier material positioned between a heat source and a heat sink in the downhole tool for reducing a flow of heat from the heat sink to the heat source. In another particular illustrative embodiment the thermal rectifier material allows more heat to flow in a first direction away from the heat source and toward the heat sink, than in a second direction away from the heat sink and toward the heat source. The thermal rectifier material may be formed into a plate or other configuration for fitting into the structure of the downhole tool.

In another particular illustrative embodiment the apparatus further includes a cooling device for transferring heat from the heat source through the thermal rectifier material. In another particular illustrative embodiment the cooling device is a thermoelectric cooling device that is electrically pulsed off and on so as to reduce the amount of heat generated by the thermoelectric cooling device itself (which it also has to pump away). In another particular illustrative embodiment the thermal rectifier material comprises a plurality of asymmetric nanotubes having their axes aligned along a heat flow path between the heat source and the heat sink. In another particular illustrative embodiment the nanotubes are carbon and boron nanotubes, unevenly coated with a heavy platinum coating.

In another particular illustrative embodiment the thermal rectifier material surrounds a liquid supply, a phase change material or an insulating flask wherein the thermal rectifier material allows more heat to flow through the thermal rectifier in a first direction toward the liquid supply, phase change material or insulating flask than in a direction through the thermal rectifier material away from the liquid supply, phase change material or insulating flask. In another particular illustrative embodiment the liquid supply is used generate vapor to transmit heat from a heat source through a vapor passage in a sorption cooling system. In another illustrative embodiment, the phase change material insulates the insulating flask.

In another particular illustrative embodiment a method for controlling heat flow in a downhole tool is disclosed. The method includes conducting a first quantity of heat through a thermal rectifier material along a heat flow path in a first direction away from a heat source and toward a heat sink; and conducting a second quantity of heat along the heat flow path, wherein the second quantity of heat is less than the first quantity of heat and wherein the second quantity of heat flows away from the heat sink and toward the heat source in a second direction. In another particular illustrative embodiment the thermal rectifier material further comprises a plurality of nanotubes having a plurality of axes aligned along the heat flow path. In another particular illustrative embodiment the thermal rectifier material surrounds a liquid supply in a sorption cooling system. In another particular illustrative embodiment the thermal rectifier material is positioned between a heat source and a heat sink. In another particular illustrative embodiment the heat source is a thermoelectric cooling device pumping heat from downhole electronics. In another particular illustrative embodiment the method includes pulsing power to the thermoelectric cooling device cyclically off and on.

In another particular illustrative embodiment a system for controlling heat flow is disclosed. The system includes a downhole tool; and a thermal rectifier material positioned between a heat source and a heat sink in the downhole tool for reducing a flow of heat from the heat sink to the heat source. In another particular illustrative embodiment the thermal rectifier material is a formed into a plate that allows more heat to flow in a first direction away from the heat source and toward the heat sink, than in a second direction away from the heat sink and toward the heat source. In another particular illustrative embodiment the system further includes a cooling device for transferring heat from the heat source through the thermal rectifier. In another particular illustrative embodiment the cooling device is a thermoelectric cooling device that is electrically pulsed off and on. In another particular illustrative embodiment the thermal rectifier material comprises a plurality of nanotubes having their axes aligned along a heat flow path between the heat source and the heat sink.

A thermal rectifier is a material that has higher thermal conductivity in one direction than it does in the opposite direction. This anisotropic behavior does not violate the laws of thermodynamics because heat still flows “downhill,” that is, from a hotter region towards a colder region. For a thermal rectifier, heat flows “downhill” more easily in one direction than it does in the opposite direction. See, for example, Chang et al. in Solid State Thermal Rectifier, Science, 17 Nov. 2006, vol. 314, no. 5802, pp. 1121-1124. Thermal rectifier phenomenon can be manifest in a rectifier material embodying carbon and/or boron nitride nanotubes that have unevenly coated with a heavy platinum compound. Along the nanotube axis, heat flowed 7% better from the high-mass and the low-mass end than along the nanotube axis in the opposite direction. The thermal rectifier material technology is still in its infancy and improvements are expected in the future. Earlier work, which did not involve nanotubes, measured a forward thermal conductivity that was as much as 210% higher than reverse thermal conductivity (a factor of 2.1 corresponding to a “directional index” of 0.526) for a thermal rectifier that was assembled from a stack of a 100 thin (56 micron) stainless steel disks of a particular surface roughness in conjunction with a solid aluminum tube.

Various scientists and engineers have chronicled progress in the manufacture and design of thermal material and thermal rectifier devices. See, for example, Solid State Thermal Rectifier by C. W. Change, D. Okawa, A. Majumdar, and A. Zetti in Science 17 Nov. 2006, Vol. 314, No. 5802, pp. 1121-1124, which is hereby incorporated herein by reference in its entirety; Electronic Nuisance Changes Its Ways by Robert F Service in Science 17 Nov. 2006, Vol. 314, No. 5802, pp. 1065-106, which is hereby incorporated herein by reference in its entirety; Differential Expansion Thermal Rectifier by A. Jones, P. W. O'Callaghan and S. D. Probert in Journal of Physics E: Scientific Instruments, 1971, pp. 438-440, which is hereby incorporated herein by reference in its entirety; A Thermal Rectifier by P. W. O'Callaghan, S. D. Probert and A. Jones in Journal of Physics D: Applied Physics, 1970, Vol. 3, pp. 1352-1358, which is hereby incorporated herein by reference in its entirety; Heat Conduction in One-Dimensional Yukawa Chains by Bambi Hu, Haibin Li and Bai-Song Xie, arXiv:cond-mat/0312058, Vol. 1, 2 Dec. 2003, which is hereby incorporated herein by reference in its entirety; Thermal Diode: Rectification of Heat Flux by Baowen Li, Lei Wang, and Giulio Casati in The American Physical Society, Volume 93, Number 18, October 2004, pp. 184301-1-184301-4, which is hereby incorporated herein by reference in its entirety; Heat Conduction in One Dimensional Systems: Fourier Law, Chaos, and Heat Control by Giulio Casati and Baowen Li, Chapter 1 of Non-Linear Dynamics and Fundamental Interactions edited by Davron Matrasulov, Faqir Khanna, arXiv:cond-mat/0502546, Vol. 1, 23 Feb. 2005, pp. 1-15, which is hereby incorporated herein by reference in its entirety; Anomalous Heat Conduction and Anomalous Diffusion in Nonlinear Lattices, Single Walled Nanotubes, and Billiard Gas Channels by Baowen Li, Jiao Wang, Lei Wang and Gang Zhang, arXiv:cond-mat/0410355, Vol. 1, 14 Oct. 2004, pp. 1-15, which is hereby incorporated herein by reference in its entirety; Spin-Boson Thermal Rectifier by Dvira Segal and Abraham Nitzan in The American Physical Society, Physical Review Letters, Vol. 94, 28 Jan. 2005, pp. 034301-1-034301-4, which is hereby incorporated herein by reference in its entirety; Heat Conduction in One-Dimensional Lattices With On-Site Potential by A. V. Savin and O. V. Gendelman in The American Physical Society, Physical Review E 67, 2003, pp. 041205-1-041205-12, which is hereby incorporated herein by reference in its entirety; Thermal Diode: Rectification of Heat Flux by Boawen Li, Lei Wang and Giulio Casati in Physical Review Letters, Vol. 93, Number 18, 29 Oct. 2004, pp. 1-4, which is hereby incorporated herein by reference in its entirety; Heat Conduction in a One-Dimensional Chain of Hard Disks with Substrate Potential by O. V. Gendelman and A. V. Savin in the American Physical Society, Physical Review Letters, Volume 92, Number 7, 20 Feb. 2004, pp. 074301-1-074301-4, which is hereby incorporated herein by reference in its entirety.

In a particular illustrative embodiment, a thermoelectric (TE) cooling device can be used to cool or remove heat from heat sources, such as electronics and detectors downhole. Each TE cooling device can be configured as a thin plate that pumps or removes heat from one face of the TE to an opposite face of the TE when the TE is supplied with electrical power. The TE can be cyclically pulsed off and on to save power and to reduce self heating, which must also be pumped away. Since a TE cooling device can exhibit a relatively high thermal conductivity between its two faces, as soon as the power is turned off to the TE cooling device, heat rapidly flows back or returns from the heat sink to the electronic part that is being cooled by the TE. Therefore, a thermal rectifier is used so that the TE cooling device does not have to be run continuously and instead can be run in a cyclical pulsed on and off mode in which the power to the TE is cycled on and off. The thermal rectifier material is positioned so that it is oriented to allow heat to flow more easily in a first direction away from the TE cooling device toward the heat sink than in a second direction away from the heat sink and toward the TE cooling device.

Running the TE cooling device continuously not only uses more power that cycling the TE cooling device on and off, but running continuously also generates more heat within the TE cooling device and that heat also has to be pumped away by the TE cooling device, thereby adding to the heat load and reducing the TE cooling device's overall effectiveness. Thus, in an illustrative embodiment a thermal rectifier material or device configured as a plate (such as one constructed of similarly-oriented parallel nanotubes, as in Chang et al., or configured as another suitable thermal rectifier assembly of materials) is placed between the TE cooling device and the heat sink, then, once the heat is pumped out of a device such as the downhole electronics and into a heat sink, it would be more difficult for heat to flow in the opposite direction back from the heat sink to the device (assuming that the rectifier was oriented to have lower thermal conductivity for the return trip). That is why conceptually, a thermal rectifier material is thought of as being a thermal “check valve” for which it is easy to pump heat out but hard for pumped-out heat to return.

Electronic components can be somewhat insulated from heat down hole using electronic insulator flasks, such as a Dewar flask. Electronic thermal insulator flasks utilize high thermal capacity materials in an attempt to insulate the electronics inside the insulator flask from heat and thus retard heat transfer from the borehole into an electronics chamber inside the insulator flask. Designers also place thermal insulators adjacent to the electronics to retard the increase in temperature caused by heat entering the flask and heat generated within the flask by the electronics. The design goal is to keep the ambient temperature inside of the electronics chamber flask below the critical temperature at which electronic failure may occur. Designers seek to keep the temperature below critical for the duration of the logging run, which is usually less than 12 hours.

Electronic container flasks, unfortunately, can take as long to cool down as they take to heat up. Thus, once the internal flask temperature exceeds the critical temperature for the electronics, it requires many hours to cool down before an electronics flask can be used again safely. In a particular embodiment, a thermal rectifier material is positioned adjacent or surrounding the electronic container flask and oriented so that heat escapes the flask more easily than heat enters the flask. In another particular embodiment, a thermoelectric cooling device is provided to cool electronics and/or a component that removes heat from the flask or electronics/sensor region without requiring extremely long cool down cycles which impede downhole operations.

In a particular illustrative embodiment an apparatus and method are disclosed for a downhole tool component cooling system using a thermal rectifier material. In a particular illustrative embodiment, a downhole tool component cooling system uses a sorption cooling system that does not require an external electrical power source. The sorption cooling system of a particular illustrative embodiment utilizes the potential energy of sorption to remove heat from a temperature sensitive tool component. The sorption system removes heat from a tool component, such as downhole electronics, and moves the exiting heat to a second, hotter region in the downhole tool.

In a particular illustrative embodiment, a thermal rectifier material allows less returning heat to flow back into the tool component from the hotter region. In one particular illustrative embodiment, a cooling region of the tool, adjacent to the temperature-sensitive component or electronics to be sorption cooled, contains a liquid source (such as water) which in the present example is a solid form of water to avoid spillage. The solid source of water releases its water as its temperature increases. Thus, this solid source of water can be a low-temperature hydrate, desiccant, sorbent, or polymeric absorber from which water (or some other liquid) vapor is generated when heated sufficiently. For example, sodium polyacrylate is a polymeric water absorber that can absorb up to 40 times its weight in water and still appear to be a dry solid.

Sorption cooling occurs as a first portion of the solid source of liquid or water releases water or another liquid vapor. Upon release from the first portion of the solid source of water or liquid, the remaining portion of this solid source of water or liquid is cooled, and this remaining portion in turn cools the adjacent thermally sensitive component (i.e., electronics), thereby keeping the adjacent component within a safe operating temperature with continued sorption cooling. Thus, an illustrative embodiment provides a structure and method whereby the downhole electronics or other thermally-sensitive components are surrounded by or adjacent to a solid source of water, such as a low-temperature hydrate, desiccant, sorbent, polymeric absorber or some mixture of these. The solid source of liquid or water surrounding or adjacent to the electronics or thermally sensitive component is cooled by release of the water vapor (or other liquid vapor), thereby cooling the electronics or other thermally-sensitive component, e.g., a sensor.

In a particular illustrative embodiment, a thermal rectifier material is provided to enhance the performance and utility of a sorbent or sorption cooling system. In a particular embodiment, thermal rectifier material in combination with a thermoelectric cooling device or a sorbent cooling system for use in a downhole tool deployed in a well, such as downhole tool in a drill string through which a drilling fluid flows, or a wire line may include a thermal, but is not limited to, one of more of the following components: (i) a tool housing adapted to be disposed in a well and exposed to the fluid in the well, (ii) a solid source of liquid (e.g., a low-regeneration-temperature hydrate, desiccant, sorbent, or polymeric absorber that releases water when heated), adjacent to a thermally sensor or electronic component to be cooled (iii) optionally, a Dewar flask lined with phase change material surrounding the electronics/sensor and liquid supply, (iv) optionally, a vapor passage for transferring vapor from the liquid supply; and (v) a high-temperature sorbent or desiccant in thermal contact with the housing for receiving and adsorbing the water vapor from the vapor passage and transferring the heat from the water vapor through the housing to the drilling fluid or wellbore.

A desiccant is a specific type of sorbent, that is a substance that sorbs (adsorbs or absorbs) water. All desiccants are sorbents but not all sorbents are desiccants. The electronics or sensor adjacent to the low-temperature hydrate, desiccant, or sorbent is kept cool by the latent heat of fusion and heat of desorption. The thermal rectifier material allows heat to easily pass through it exiting the Dewar flask, but allows heat to flow less easily through it entering the Dewar flask. Thus, because of the surrounding thermal rectifier material, the phase change liquid supply heats up more slowly during down hole operations and cools down more rapidly when returned to the surface, which enables the phase change material to resolidify more quickly for reuse in another run downhole.

A drilling operation according one particular illustrative embodiment is shown in FIG. 1. A drilling rig 1 drives a drill string 3 that, which typically is comprised of a number of interconnecting sections. A downhole assembly 11 is formed at the distal end of the drill string 3. The downhole assembly 11 includes a drill bit 7 that advances to form a bore 4 in the surrounding formation 6. A portion of the downhole assembly 11, incorporating an electronic system 8 and cooling systems according to a particular illustrative embodiment is shown in FIG. 2. Turning now to FIG. 2, the electrical system 8 may, for example, provide information to a data acquisition and analysis system 13 located at the surface. The electrical system 8 includes one or more electronic components. Such electronic components include those that incorporate transistors, integrated circuits, resistors, capacitors, and inductors, as well as electronic components such as sensing elements, including accelerometers, magnetometers, photomultiplier tubes, and strain gages.

The downhole portion 11 of the drill string 3 includes a drill pipe, or collar, 2 that extends through the bore 4. As is conventional, a centrally disposed passage 20 is formed within the drill pipe 2 and allows drilling mud 22 to be pumped from the surface down to the drill bit. After exiting the drill bit, the drilling mud 23 flows up through the annular passage formed between the outer surface of the drill pipe 2 and the internal diameter of the bore 4 for return to the surface. Thus, the drilling mud flows over both the inside and outside surfaces of the drill pipe. Depending on the drilling operation, the pressure of the drilling mud 22 flowing through the drill pipe internal passage 20 will typically be between 1,000 and 20,000 pounds per square inch, and, during drilling, its flow rate and velocity will typically be in the 100 to 1500 GPM range and 5 to 150 feet per second range, respectively.

As also shown in FIG. 2, the electrical system 8 is disposed within the drill pipe central passage 20. The electrical system 8 includes a number of sensor modules 10, a control module 12, a power regulator module 14, an acoustic pulser module 18, and a turbine alternator 16 that are supported within the passage 20, for example, by struts extending between the modules and the drill pipe 2. According to the current invention, power for the electrical system 8, including the electronic components and sensors, discussed below, is supplied by a battery, a wireline or any other typical power supply method such as the turbine alternator 16, shown in FIG. 2, which is driven by the drilling mud 22. The turbine alternator 16 may be of the axial, radial or mixed flow type. Alternatively, the alternator 16 could be driven by a positive displacement motor driven by the drilling mud 22, such as a Moineau-type motor. In other embodiments, power could be supplied by any power supply apparatus including an energy storage device located downhole, such as a battery.

As shown in FIG. 3, each sensor module 10 is comprised of a cylindrical housing 52, which in an illustrative embodiment is formed from stainless steel or a beryllium copper alloy. An annular passage 30 is formed between the outer surface 51 of the cylindrical housing 52 and the inner surface of the drill pipe 2. The drilling mud 22 flows through the annular passage 30 on its way to the drill bit 7, as previously discussed. The housing 52 contains an electronic component 54 for the sensor module. The electronic component 54 may, but according to a particular illustrative embodiment, does not necessarily, include one or more printed circuit boards including a processor associated with the sensing device, as previously discussed. Alternatively, the assembly shown in FIG. 3 comprises the control module 12, power regulator module 14, or pulser module 18, in which case the electronic component 54 may be different than those used in the sensor modules 10, although it may, but does not necessarily, include one or more printed circuit boards. According to a particular illustrative embodiment, one or more of the electronic components or sensors in the electrical system 8 are cooled by evaporation of liquid from the liquid supply 132 adjacent to or surrounding electronics 54.

A highly heat-conductive polymer is optionally provided proximate or touching the electronics or circuit board to facilitate heat removal from the electronics or circuit board, as shown in FIG. 4. These polymers are typically loaded with highly heat-conductive minerals. At room temperature, they feel quite cool to the touch because they quickly draw heat from one's fingers. Water is a particularly effective coolant for use in a sorption cooling system. Evaporation of one liter of water removes 631.63 Watt-hours of energy, which equals 543 cal/ml. Water is also inexpensive, readily available worldwide, nontoxic, chemically stable, and poses no environmental disposal problems. Thus, evaporation of one liter of water can remove 632 Watts for one hour, 63 Watts for 10 hours, or 6.3 Watts for 100 hours.

In a particular illustrative embodiment, a low-temperature solid source of water is placed inside the cooling region of the downhole tool, preferably inside a Dewar flask. A high-temperature desiccant that is in thermal contact with the wellbore fluid adsorbs the water released by the low-temperature solid source of water. The high-temperature desiccant is chosen based on the desired operating temperature, that is, the temperature at which a desiccant releases water.

In one particular illustrative embodiment, approximately 6.25 volumes of loosely packed high-temperature desiccant are utilized to sorb 1 volume of water. After each logging run, the high-temperature desiccant can either be discarded or regenerated. This higher temperature desiccant can be regenerated by heating it to the water release temperature to release the water or other liquid it has absorbed by the higher temperature desiccant during sorption cooling. Some sorbents, referred to as desiccants, are able to selectively sorb water. Some desiccants retain sorbed water even at relatively high temperatures. Molecular Sieve 3A (MS-3A), and 13× are synthetic zeolites that are high-temperature desiccants. The temperature for desiccant regeneration or expulsion of sorbed water for MS-3A ranges from 175° C. to 350° C.

Turning now to FIG. 4 a wire line deployment of the present invention is depicted. FIG. 4 schematically depicts a well bore 101 extending into a laminated earth formation, into which well bore a logging tool including sensors and electronics as used according to the present invention has been lowered. The well bore in FIG. 4 extends into an earth formation which includes a hydrocarbon-bearing sand layer 103 located between an upper shale layer 105 and a higher conductivity than the hydrocarbon bearing sand layer 103. An electronic logging tool 109 having sensors and electronics and a sorption or thermal conductive cooling system, has been lowered into the well bore 101 via a wire line 111 extending through a blowout preventer 113 (shown schematically) located at the earth surface 115. The surface equipment 122 includes an electric power supply to provide electric power to the set of coils 118 and a signal processor to receive and process electric signals from the sensors and electronics 119. Alternatively, a power supply and signal processor are located in the logging tool. In the case of the wire line deployment, the wire line may be utilized for provision of power and data transmission.

Turning now to FIG. 5, in one particular illustrative embodiment, electronics 502 act as a heat producing heat source and thus are positioned adjacent thermal rectifier material 506. The thermoelectric cooling device 504 is positioned between thermal rectifier material 506 and heat sink 508. The heat sink is adjacent tool housing exterior which adjoins borehole fluid 501, thus transferring heat from the heat sink to the borehole fluid. Exiting heat 514 flows more easily through the thermal rectifier material 506 in a direction from electronics 502 through thermoelectric cooling device 504 toward the heat sink 508. As shown in FIG. 5, the thermal electric cooling device is in a pulsed on mode so that power is cyclically applied on and off to the thermoelectric cooling device so that the thermoelectric cooling device pumps exiting heat 514 from electronics or heat source 502 toward heat sink 508 through thermal rectifier material 506. In another particular embodiment, the thermoelectric cooling device is placed adjacent the electronics and the thermal rectifier material is placed between the thermoelectric cooling device and the heat sink.

Turning now to FIG. 6, as shown in FIG. 6 the thermo electric cooling device is in an off cycle so that heat is not being pumped by the thermo electric cooling device from the electronics. The thermal electric cooling device conducts returning heat 516 so that heat flows back from the heat sink through the thermal rectifier material during the thermal electric cooling device off cycle. As shown by the relative sizes of the arrows representing exiting heat 514 (heat being pumped out of the electronics) and returning heat 516 (heat returning from the heat sink), the quantity of exiting heat is greater than the quantity of returning heat, due to the anisotropic properties of the thermal rectifier material. That is, exiting heat 514 flows through the thermal rectifier material more easily in a direction away from the electronics (heat source) toward the heat sink, than returning heat 516 flows in the opposite direction away from the heat sink and toward the electronics (heat source). This anisotropic heat flow property of the thermal electric material enables pulsed cooling of the electronics (heat source) by pulsing of the thermal electric cooling device by retarding heat flow back into the electronics during the off cycles for the thermal electric cooling device 504.

Turning now to FIG. 7, FIG. 7 is a schematic representation of another particular illustrative embodiment. As shown in FIG. 7, a thermal rectifier material 137 or device allows heat to flow out of a Dewar insulating flask 132 more easily than into the flask, thus enabling a phase change material 134 inside the flask to heat more slowly and to cool more rapidly. The thermal rectifier material enables the phase change material to return to solid form more rapidly for redeployment down hole. In a particular illustrative embodiment shown in FIG. 7, the electronics 54 or another device such as a sensor to be cooled are adjacent a solid liquid supply 131, for example a hydrate material containing water which releases water vapor 133 to cool electronics 54. The liquid supply 131 may also be positioned adjacent to electronics 54. The electronics 54 and liquid container 131 are encased and surrounded by a phase change material 134.

The phase change material acts as a temporary heat sink which retards heat flow into the chamber formed by the interior of the phase change material. The electronics 54, liquid container 131, and phase change material 134 are encased and surrounded by, in one particular illustrative embodiment, a thermally insulating Dewar flask 132. Insulating Dewar flask 132 and phase change material 134 serve as thermal insulator barriers to retard heat flow from surrounding areas into the electronics 54. The Dewar insulating flask is lined or surrounded by thermal rectifier material 137. Exiting heat 514 passes through the thermal rectifier material 137.

Vapor passage 138 runs through Dewar flask 132, phase change material 134 and liquid container 131, thereby providing a vapor escape route from liquid supply 131 to desiccant 140. As the water evaporates the water vapor 139 escapes through the vapor passage and removes heat from the adjacent to the heat source or electronics 54 or cools a similarly situated sensor. The vapor evaporates from the liquid supply 131 and passes through vapor passage 138 to desiccant 140 where the vapor is adsorbed. The liquid, preferably water, cools at it evaporates, thereby cooling electronics 54 adjacent to liquid supply 131. Desiccant 140 adsorbs water vapor thereby keeping the vapor pressure low inside of liquid container 132 and facilitating further evaporation and cooling.

In one particular illustrative embodiment, filter 135 comprises a porous rock which controls evaporation and thus controls the temperature of the liquid inside liquid supply 131 by controlling the evaporation rate of the liquid from liquid supply 131. Filter 135 controls the vapor pressure inside liquid supply 131, thereby controlling the evaporation rate from the liquid inside of liquid supply 131 by controlling the flow rate of vapor escaping from liquid supply 131. In a particular illustrative embodiment, filter 135 comprises a passive filter of porous rocks. Any suitable material which temporarily absorbs the water vapor or temporarily retards the flow of the vapor from lower passage 138 a through vapor passage 138 and releases it again to the upper portion 138 b of vapor passage 138 is a suitable filter. The filter 135 releases the vapor into the upper vapor passage 138 b where it travels through the upper vapor passage 138 b to desiccant 140. Thus, passive filter 135 limits the cooling rate of the electronics during a downhole run to avoid overcooling to an unnecessarily low temperature that would cause more rapid heat flow across Dewar walls and therefore waste water and desiccant.

Desiccant 140 is contained in desiccant chamber 142 which is in thermal contact with down tool housing 52. Downhole tool housing is in thermal contact with borehole annulus containing borehole mud 23, thereby enabling heat to flow out of desiccant chamber 142 into the bore hole. Thus, heat is removed from electronics 54, and transmitted to desiccant 140 via the liquid vapor and conducted out of the downhole tool housing 52 to the bore hole through tool housing exterior 509.

Turning now to FIG. 8, a flow chart 800 is shown, wherein in another particular illustrative embodiment, heat flow is controlled by a thermal rectifier material in a downhole tool. In a particular illustrative embodiment, the thermal rectifier material is positioned between a heat source and a heat sink for reducing the flow of heat from the heat sink to the heat source at block 802. The thermal rectifier material is configured as a plate that allows more heat to flow in a first direction away from the heat source and toward a heat sink than in a second direction away from the heat sink and toward the heat source at block 802. A thermoelectric cooling device transfers heat from the heat source (e.g., electronics) through the thermal rectifier at block 806 and pumps the heat to a heat sink. The power to the thermoelectric cooling device is cyclically pulsed off and on at block 808. The thermal rectifier material, in one particular illustrative embodiment, includes a plurality of nanotubes having axes aligned along the heat flow path between the heat source and the heat sink at block 810. In one particular illustrative embodiment, the nanotubes are carbon and boron nanotubes evenly coated with a heavy platinum coating at block 812. Numerous thermal rectifier materials are suitable for use in other illustrative embodiments. Some of the suitable thermal rectifier materials are described herein, but are not intended to be limiting as to thermal rectifier materials, which are also suitable for downhole use in other illustrative embodiments.

The foregoing example is for purposes of example only and is not intended to limit the scope of the invention which is defined by the following claims. 

1. An apparatus for controlling heat flow in a downhole tool, the apparatus comprising: A thermal rectifier material positioned between a heat source and a heat sink in the downhole tool for reducing a flow of heat from the heat sink to the heat source.
 2. The apparatus of claim 1, wherein the thermal rectifier material allows more heat to flow in a first direction away from the heat source and toward the heat sink, than in a second direction away from the heat sink and toward the heat source.
 3. The apparatus of claim 1, further comprising: a cooling device for transferring heat from the heat source through the thermal rectifier material.
 4. The apparatus of claim 4, wherein the cooling device is a thermoelectric cooling device that is electrically pulsed off and on.
 5. The apparatus of claim 1, wherein the thermal rectifier material comprises a plurality of nanotubes having their axes aligned along a heat flow path between the heat source and the heat sink.
 6. The apparatus of claim 5, wherein the nanotubes are carbon and boron nanotubes, unevenly coated with a heavy platinum coating.
 7. The apparatus of claim 1, wherein the thermal rectifier material surrounds thermal component selected from the group consisting of a phase change material, an insulating flask and a liquid supply, wherein the thermal rectifier material allows more heat to flow through the thermal rectifier material in a first direction toward the thermal component than in a direction through the thermal rectifier material away from the thermal component.
 8. The apparatus of claim 7, wherein the liquid supply is used generate vapor to transmit heat from a heat source through a vapor passage in a sorption cooling system.
 9. The apparatus of claim 7, wherein the phase change material insulates the insulating flask.
 10. A method for controlling heat flow in a downhole tool, the method comprising: conducting a first quantity of heat through a thermal rectifier material along a heat flow path in a first direction away from a heat source and toward a heat sink; and conducting a second quantity of heat along the heat flow path, wherein the second quantity of heat is less than the first quantity of heat and wherein the second quantity of heat flows away from the heat sink and toward the heat source in a second direction.
 11. The method of claim 10, wherein the thermal rectifier material further comprises a plurality of nanotubes having a plurality of axes aligned along the heat flow path.
 12. The method of claim 10, wherein the thermal rectifier material surrounds a liquid supply in a sorption cooling system.
 13. The method of claim 10, wherein the thermal rectifier material is positioned between a heat source and a heat sink.
 14. The method of claim 13, wherein the heat source is a thermoelectric cooling device pumping heat from downhole electronics.
 15. The method of claim 14, further comprising: pulsing power to the thermoelectric cooling device cyclically off and on.
 16. A system for controlling heat flow, comprising: a downhole tool; and a thermal rectifier material positioned between a heat source and a heat sink in the downhole tool for reducing a flow of heat from the heat sink to the heat source.
 17. The system of claim 16, wherein the thermal rectifier material allows more heat to flow in a first direction away from the heat source and toward the heat sink, than in a second direction away from the heat sink and toward the heat source.
 18. The system of claim 16, further comprising: a cooling device for transferring heat from the heat source through the thermal rectifier.
 19. The system of claim 18, wherein the cooling device is a thermoelectric cooling device that is electrically pulsed off and on.
 20. The system of claim 16, wherein the thermal rectifier material comprises a plurality of nanotubes having their axes aligned along a heat flow path between the heat source and the heat sink. 